Differentation without cleavage has been previously described at the cellular level under a variety of conditions. None of these reports has dealt with the possibility of DNA synthesis and gene activation in the absence of cell division of fertilized eggs. The present application proposes to investigate gene activation in cold treated cleavage arrested sea urchin eggs by means of autoradiography, during the four day period, as well as by density gradient characterization of newly synthesized RNA. As a direct assay of replication and transcription of paternal genome reciprocal hybrids between two genera will be produced. These can be cleavage arrested or not, depending on the maternal parent. The same two genomes can be manipulated under cleaving and non-cleaving conditions. The kind of DNA produced by the hybrids will be investigated by DNA:DNA reannealing procedures utilizing unique sequence DNA. The F1 histone fraction of hybrid and parental type embryos will be used as a marker for translation of embryonic template. Characterization of the pattern of DNA synthesis of the parent embryos and reciprocal hybrids at 10, 20, and 25 C has been completed using quantitative microspectrophotometry. Other pilot work includes autoradiographs of RNA and DNA synthesis by cold inhibited homospermic embryos, in which it was shown that transcription of RNA occurs in cleavage arrested embryos between 24 and 48 hours post fertilization. The labeled RNA localized in the autoradiographs is RNAse sensitive. DNA synthesis by cleavage arrested embryos has been studied by Feulgen microspectrophotometry and labelled thymidine autoradiography. The two methods agree on DNA localization, and the thymidine localization in autoradiographs is DNAse sensitive. Thus, the importance of normal cleavage pattern in gene activation and synthesis of ribosomal and heterogeneous RNA will be studied. Electronmicrograph studies will complement the biochemical work. The results are of significance in the area of the genomic control of cell differentiation.